1. Field of the invention.
Our invention relates to the field of non-destructive testing, and more particularly, to high resolution imaging of internal strains in crystal structures.
2. Description of the related art.
Semiconductor fabrication processes are requiring larger and more perfect crystals. Currently, routine integrated circuit (IC) testing is dominated by measurement of the optical and electrical material/device properties through final device performance and parametric testing. The occurrence of internal flaws, such as those which may relate even to the internal microstructure of the substrate crystal, may result in an entire production run being worthless. Obviously, flaws detected only after completion of the production run are found too late to allow for corrective action.
While methods for the characterization of internal strains or flaws within crystal microstructures are known, these methods are not yet considered sufficiently insightful or necessary to be a routine part of the IC fabrication process. Known procedures for analyzing the degree of perfection of a crystal structure include the use of double-crystal rocking curves (see X. Chu and B. K. Tanner, "Double Crystal X-Ray Rocking Curves of Multiple Layer Structures," Semicond. Sci. Technol. 2: 765, 1987), x-ray topography (see G. A. Rozogonyi and D. Miller, "X-Ray Characterization of Stresses and Defects in Thin Films and Substrates," Methods and Phenomena 2-Characterization of Epitaxial Semiconductor Films. H. Kressel, ed., Elsevier Scientific Publishing Co., 1976, p. 185), or a combination of these techniques.
The x-ray diffraction topography method is capable, in principle, of visually revealing lattice strains and misorientations over large crystal areas. Recently, it has been demonstrated that such topography information can be used to correlate lattice strain and device performance in silicon integrated circuits (see S. B. Qadri, D. Ma, and M. Peckerar, "Double-Crystal X-ray Topographic Determination of Local Strain in Metal-Oxide Semiconductor Device Structures," Appl. Phvs. Lett. 51: 1827 [1987]). Many different x-ray topography geometries have been used to date, and each system has its own advantages. The conventional Asymmetric Crystal Topography (ACT) system gives excellent measurements of small angular misorientations potentially occurring in relatively large, nearly perfect single crystals (see W. J. Boettinger, H. E. Burdette, M. Kuriyama, R. E. Green, Jr., "Asymmetric Crystal Topographic Camera," Rev. Sci. Instrum., v. 47, No. 8, August 1976, p. 906). Another topography technique is known to reveal local dislocation images with excellent spatial resolution (A. R. Lang, Diffraction and Imaging Techniques, v. 2, North-Holland, Amsterdam, 1978, p. 678). Superior resolution of individual dislocations and their accumulated strain fields has been obtained using the Berg-Barrett topography technique in a configuration with a limiting small specimen to recording media distance, as specified by Newkirk (J. B. Newkirk, "The Observation of Dislocations and other Imperfections by X-ray Extinction Contrast," Trans. Metallurqical Soc. AIME, v. 215, June 1959, p. 431).
A detailed analysis of semiconductor material and associated integrated circuits is imperative for ensuring quality products. To date, there is no effective nondestructive technique which has both high vertical and high horizontal resolution and which can be used after each step in the production process and still allow the wafer to be reinserted for further processing. A desirable process must provide fine angular resolution in diffraction images of varying crystal orientations spread over large areas. It must have optimum spatial resolution over a large area, with minimal time required for equipment set-up and exposure of the recording media. Finally, high resolution must be obtainable with reasonable working distances between an inspection-beam-forming x-ray monochromating crystal, the crystal specimen to be examined, and the image recording media capturing, non-destructively, a picture of the internal crystal strains and misorientations.